In recent years, a solid electrolytic capacitor has been requiring the decrease in size and the increase in capacitance, and a solid electrolytic capacitor using niobium oxide having a large dielectric constant have been proposed, instead of using aluminum oxide or tantalum oxide as a dielectric material. An example of such proposal includes Japanese published unexamined patent application No. 2000-68157.
FIG. 8 is a cross-sectional view for illustrating a structure of a conventional solid electrolytic capacitor. The structure of the conventional solid electrolytic capacitor will be described in reference to FIG. 8.
As shown in FIG. 8, in the conventional solid electrolytic capacitor, an anode 101 comprises an anode lead 101a made of tantalum and a rectangular block shaped base body 101c comprised of a porous sintered body made from niobium particles, and the anode lead 101a is partially embedded in the base body 101c. 
Onto the anode 101, an oxide layer 102 is formed so as to cover an area surrounding the base body 101c. The oxide layer 102 functions as a dielectric layer.
Onto the oxide layer 102, an electrically conductive polymer layer 103 made of polypyrrole or the like is formed so as to cover an area surrounding the oxide layer 102. The electrically conductive polymer layer 103 functions as an electrolyte layer. Onto the electrically conductive polymer layer 103, a cathode 104 is formed, which is laminated with a first electrically conductive layer 104a containing carbon particles formed so as to cover an area surrounding the electrically conductive polymer layer 103 and a second electrically conductive layer 104b containing silver particles formed so as to cover an area surrounding the first electrically conductive layer 104a. 
Onto the top surface of an area surrounding the cathode 104, an electrically conductive adhesive layer 105 is formed, onto which a cathode terminal 106 is further formed. An anode terminal 107 is connected onto the anode lead 101a, which is exposed from the base body 101c and the oxide layer 102. Further, a mold-packaging resin 108 is formed around the cathode 104, the cathode terminal 106 and the anode terminal 107 such that respective edges of the cathode terminal 106 and the anode terminal 107 can be led outside. The conventional solid electrolytic capacitor is thus configured.
FIGS. 9 to 13 are cross-sectional views for illustrating a fabrication process of the conventional solid electrolytic capacitor. In reference to FIGS. 9 to 13, the fabrication process of the conventional solid electrolytic capacitor having the structure configured as above is now described.
First, as shown in FIG. 9, the anode 101 comprising the anode lead 101a made of tantalum and the rectangular block shaped base body 101c comprised of the porous sintered body made from niobium particles is formed. The base body 101c is formed by heat treatment process in vacuum a molding made from niobium particles, in which the anode lead 101a is partially embedded.
Then, the anode 101 is anodized in an aqueous solution such as a phosphoric acid solution to thereby form the oxide layer 102 on the base body 101c so as to cover the area surrounding the base body 101c as shown in FIG. 10.
Subsequently, as shown in FIG. 11, the electrically conductive polymer layer 103 made of polypyrrole or the like is formed onto the oxide layer 102 by polymerization or the like so as to cover the area surrounding the oxide layer 102.
Subsequently, as shown in FIG. 12, a carbon paste is coated onto the electrically conductive polymer layer 103 so as to cover the area surrounding the electrically conductive polymer layer 103, and then dried to thereby form the first electrically conductive layer 104a containing carbon particles. After that, a silver paste is coated on the first electrically conductive layer 104a so as to cover the area surrounding the first electrically conductive layer 104a, and then dried to thereby form the second electrically conductive layer 104b containing silver particles. This allows the cathode 104, which is laminated with the first electrically conductive layer 104a and the second electrically conductive layer 104b, to be formed so as to cover the area surrounding the electrically conductive polymer layer 103.
Subsequently, as shown in FIG. 13, after an electrically conductive adhesive has been coated onto the cathode terminal 106, the cathode terminal 106 is bonded to the top surface of the area surrounding the cathode 104 through the electrically conductive adhesive. Further, drying the electrically conductive adhesive allows the electrically conductive adhesive layer 105 to be formed, through which the cathode 104 and the cathode terminal 106 are interconnected. Also, the anode terminal 107 is welded onto the anode lead 101a exposed from the base body 101c and the oxide layer 102.
Finally, as shown in FIG. 8, the mold-packaging resin 108 is formed around the cathode 104, the cathode terminal 106 and the anode terminal 107 such that the respective edges of the cathode terminal 106 and the anode terminal 107 can be led outside.
FIG. 14 is a schematic cross-sectional view for illustrating a condition of the oxide layer near the anode lead in the conventional solid electrolytic capacitor. Referring to FIG. 14, in the conventional solid electrolytic capacitor, the oxide layer 102 is formed on the surface of the anode lead 101a at positions of spaces that exist between the anode lead 101a and the base body 101c due to the base body 101c comprised of the porous sintered body, as well as on the surface of the base body 101c. Note that because the anode lead 101a is made of tantalum, an oxide layer 102a made of tantalum oxide is formed on the anode lead. On the other hand, because the base body 101c is made of niobium, an oxide layer 102b made of niobium oxide is formed on the base body 101c. For this reason, in the spaces existing between the anode lead 101a and the base body 101c, the oxide layer 102 is in a condition of a mixture of the oxide layer 102a made of tantalum oxide and the oxide layer 102b made of niobium oxide.
The above conventional solid electrolytic capacitor can achieve large electrostatic capacitance because the oxide layer 102 is comprised of niobium oxide except for an area in the vicinity of the interface between the anode lead 101a and the base body 101c as described.
However, in the above conventional solid electrolytic capacitor, the oxide layer 102 in the vicinity of the interface between the anode lead 101a and the base body 101c is in the condition of the mixture of the oxide layer 102a made of tantalum oxide and the oxide layer 102b made of niobium oxide, and therefore stress due to the difference in thermal expansion coefficient between the oxide layer 102a made of tantalum oxide and the oxide layer 102b made of niobium oxide arises during heat treatment process such as reflow soldering process. For this reason, there have arisen problems that the oxide layer 102 is likely to be peeled off from the anode lead 101a and the base body 101c and that cracks are likely to be generated inside of the oxide layer 102. As a result, there has arisen another problem that leakage current between the cathode 104 formed on the oxide layer 102 and the anode 101 is likely to be increased.
The present invention has been made in order to solve the problems described above, and it is therefore one object of the present invention to provide a solid electrolytic capacitor element or a solid electrolytic capacitor having low leakage current.
Another object of the present invention is to provide a method for manufacturing the solid electrolytic capacitor element having low leakage current.